Heat pump systems used for air conditioning and heating commonly consist of indoor units combined with outdoor units to provide heating and cooling for indoor spaces. Cooling and heating is achieved through a vapor compression cycle. Heat is absorbed from the indoor space (cooling the space) through the indoor unit and discharged to the outdoors at the outdoor unit. Heating is achieved by reversing the cycle. Heat is absorbed from the outside at the outdoor unit and discharged to the inside through the indoor units.
For air-source based heat pumps, the cooling operation tends to become unstable and system capacity starts to drop off as the ambient air temperature drops below a low ambient threshold temperature, e.g., below 23° F. (−5° C.). Wind blowing against the coil surfaces, which tend to be exposed to ambient conditions, can exacerbate the negative affect a drop in ambient air temperature below the low ambient threshold temperature has on system stability and capacity.
Attempts to combat the negative affects of low ambient air temperature and wind have included installing wind guards that actually restrict the airflow to allow a lower outdoor operating temperature. In such designs, however, the restriction tends to be constant which negatively impacts capacity both when cooling at higher outdoor temperatures and during the heating mode.
It is desirable to provide systems and methods that facilitate low ambient air temperature cooling in air-source based heat pump systems.